The present invention relates to an improved process for producing high-purity 2,2-bis(4hydroxyphenyl)propane (referred to as bisphenol A hereinafter) in such a manner that the formation of by-products is suppressed.
Bisphenol A is a raw material for polycarbonate resins and epoxy resins. Bisphenol A used for polycarbonate resins is required to be colorless and highly pure.
Bisphenol A is produced by the condensation reaction of phenol with acetone in the presence of an acid catalyst and an optional co-catalyst such as a sulfur compound. The reaction product mixture contains, in addition to bisphenol A, the catalyst, unreacted acetone, unreacted phenol, water and by-products.
The by-products contain as major components 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (referred to as o,p'-isomer hereinafter) and Dianin's compound. Minor components include 2,4-di-[2-(4-hydroxyphenyl)isopropyl]-phenol (referred to as trisphenol hereinafter), polyphenols, and substances which cause discoloration of the end product. They have adverse effects on the performance of the resins produced from such a bisphenol A.
Examples of acidic catalysts for the condensation reaction include inorganic acids (such as hydrochloric acid) and strongly acidic ion-exchange resins.
The reaction which employs hydrochloric acid as a catalyst may be carried out at a low temperature so that the adduct of bisphenol A and phenol crystallizes out of the reaction product mixture as the reaction proceeds. The reaction in this manner is used for the production of high-purity bisphenol A because the o,p'-isomer, which is a major by-product, is isomerized into bisphenol A or p,p'-isomer during the reaction so that the formation of the by-product, o,p'-isomer can be reduced.
The formation of Dianin's compound, which is another major by-product, can be reduced to some extent by adding a mercapto compound to the reaction mixture, as disclosed in Japanese Patent Publication No. 5367/1952. However, this is not practical for industrial use because the mercapto compound needs a complicated procedure for separation thereof and gives an undesirable odor to the product. The formation of Dianin's compound can also be reduced by using excess phenol. However, this is uneconomical because the excess phenol has to be separated from bisphenol A eventually. Another disadvantage is an increased formation of the o,p'-isomer.
In the reaction that employs hydrochloric acid as a catalyst, it is possible to obtain high-purity bisphenol A if the catalyst concentration is increased, as disclosed in Japanese Patent Publication No. 7186/1965 and GB No. 1052618. According to the latter disclosure, hydrogen chloride should be fed to the reaction zone under a pressure of 3.5 kg/cm.sup.2 or above. According to the former disclosure, the reaction starts with a mixture of reactants (acetone and phenol) and water (3 to 10 wt % of the reactants) and proceeds while the reaction zone is being supplied with hydrogen chloride gas in an amount sufficient to saturate the water in the reaction zone.
There is disclosed in Japanese Patent Laid-open No. 93347/1974 a method of increasing the reaction rate by adding water to the phenol-acetone liquid layer in an amount slightly more than the amount necessary to attain saturation. A disadvantage of this method is that the reaction needs excess hydrogen chloride and a large amount of energy is required for the removal of hydrogen chloride, water, and phenol from the reaction product mixture and the recovery of the desired product from the mixture of phenol, water, and hydrogen chloride. This leads to an increased production cost for the bisphenol A. Another disadvantage of adding water to the reaction mixture is that the solubility of bisphenol A in phenol increases in the presence of water, with the result that the isomerization reaction decreases and hence the ratio of o,p'-isomer to bisphenol A increases.